This invention relates to volatile memory elements, and more particularly, to volatile memory elements that operate at elevated power supply levels on integrated circuits such as programmable logic device integrated circuits.
Integrated circuits often contain volatile memory elements. Typical volatile memory elements are based on cross-coupled inverters (latches) and are used to store data. Each memory element can store a single bit of data.
Volatile memory elements are often used to store configuration data in programmable logic devices. Programmable logic devices are a type of integrated circuit that can be customized in relatively small batches to implement a desired logic design. In a typical scenario, a programmable logic device manufacturer designs and manufactures uncustomized programmable logic device integrated circuits in advance. Later, a logic designer uses a logic design system to design a custom logic circuit. The logic design system uses information on the hardware capabilities of the manufacturer's programmable logic devices to help the designer implement the logic circuit from the resources available on a given programmable logic device.
The logic design system creates configuration data based on the logic designer's custom design. When the configuration data is loaded into the memory elements of one of the programmable logic devices, it programs the logic of that programmable logic device so that the programmable logic device implements the designer's logic circuit. The use of programmable logic devices can drastically reduce the amount of effort required to implement a desired integrated circuit design.
Conventional programmable logic device memory elements are powered using a positive power supply voltage and a ground. The positive power supply voltage that is used to power conventional programmable logic device memory elements is typically referred to as Vcc or Vcc-core and is the same power supply voltage used to power the core logic in the programmable logic device. The ground voltage that is used to power conventional programmable logic device memory elements is referred to as Vss and is the same ground power supply voltage that is used in powering the core logic.
Integrated circuits such as programmable logic device integrated circuits that operate at low values of Vcc offer benefits over integrated circuits that operate at higher values of Vcc. For example, reductions in Vcc generally lead to reduced power consumption. Because of these benefits, the semiconductor industry is continually striving to produce processes and circuit designs that support reductions in Vcc. Previous generations of programmable logic devices operated at Vcc levels of 2.0 volts, 1.8 volts, and 1.5 volts. More recently, Vcc levels of 1.2 volts have been used in programmable logic devices. It is expected that future programmable logic devices will support Vcc levels of less than 1.2 volts (e.g., 1.1 volts or 1.0 volts).
The memory elements in a programmable logic device produce static output signals that reflect the configuration data that has been loaded into the memory elements. The static output signals drive the gates of metal-oxide-semiconductor (MOS) transistors. Some of the transistors are n-channel metal-oxide-semiconductor (NMOS) transistors that are used as pass transistors in multiplexers and other logic components. Other transistors are p-channel metal-oxide-semiconductor (PMOS) transistors that are used as power-down transistors that selectively turn off unused blocks of circuitry.
The NMOS pass transistors in a programmable logic device will only operate properly if they are driven at a large enough voltage. If the pass transistors in a programmable logic device are driven at an insufficient voltage level because, for example, the Vcc level on the device is too low, the data signals passing through the pass transistors will suffer excessive voltage loss and may no longer be recognizable as valid logic signals on the device.
The PMOS power-down transistors will only operate satisfactorily if they can be turned off completely. If a PMOS transistor is turned of insufficiently, undesirable leakage currents will be produced.
It would therefore be desirable to be able to provide programmable logic device integrated circuits whose NMOS pass transistors and PMOS power down transistors perform satisfactorily, particularly in operating environments with low core logic power supply voltages.